2-cyano-6-oxybenzothiazole and methods of preparing same



United States Patent 6 Claims. Cl. 260-304 It has been obtained fromextracts of the lantern of the American firefly Photinus pyralis where,presumably in combined form, it is an important factor in the lightemission of the firefly. Heretofore no methods were known for thechemical synthesis of this benzothiazole. It is an object of the presentinvention to provide such a chemical synthesis, and to provide novelchemical compounds which are key intermediates therein. It is a furtherobject to provide 2-substituted benzothiazoles having the generalstructural formula N N C O 011 Yitltri;

Where x is C(CH CH or CH -CH X and Y are H or OH and one of X and Y isH. A still further object is provision of chemical methods of makingsuch compounds.

According to the present invention, it has now been found that2-substituted-6-hydroxybenzothiazoles and 2-su'bstituted-4-hydroxybenzothiazoles may be produced from a lower alkylester of N-(4-loweralkoxy)phenyl oxamic acid or N-(2-loweralkoxy)phenyloxamic acid by the process represented below:

H H N-C-COOR N-(ITOOOH s B- B II III L it 000R B S 00011 B 8 IV VPatented July 26, 3966 VI VII N N N 00011 T ill iii/ ta In the aboveflow chart, R represents a lower alkyl radical, A and B are hydrogen orlower alkoxy groups, and A and B are diiferent in any given compound, Xand Y are hydrogen or hydroxy, and X and Y are different in a particularcompound, and x is CH C(CH or CH CH Thus, compounds H through VII haveone lower alkoxy group in the six mernbered ring, which group is in the4- or 6-position of the benzothiazole nucleus. Compounds I and VIH havea hydroxy group in either the 4- or 6-position of the benzothiazole ringsystem.

The following detailed description of our invention will, for the sakeof clarity and convenience, be concerned primarily with the preparationof 2-cyano-6-hydroxybenzothiazole (VIII, X=H, Y=OH), from lower alkylN-(4- methoxy) phenyl oxamate (II, A=H, B=OCH and with the conversion of2-cyano-6-hydroxybenzothiazole into 2-[2 (4-carboxy)thiazolinyl]-6-hydroxybenzothiazole (I, X=H, Y==OH, x= CH It is to beunderstood, however, that the methods described for making thesecompounds apply also to the synthesis of the other substances includedwith the above flow sheet.

In the first step of our process a lower alkyl N-(4- methoxy)pheny1oxarnate, such as methyl, ethyl or propyl N-(4-methoxy)phenyl oxamate,is converted to the corresponding thiooxamic acid by reaction withphosphorus pentasulfide and treatment of the resulting product with abase, and preferably with an alkali metal hydroxide such as sodium orpotassium hydroxide. The reaction is preferably carried out at elevatedtemperatures of from about C. to about C. in an inert organic solventsuch as benzene, toluene or xylene. Reaction times of from 30 minutes to2 hours are normally adequate for optimum yields. It is convenient tofollow the course of the reaction by measuring the change in ultravioletspectrum of the products. The starting material has an absorption peakin the ultraviolet spectrum at 283 m Whereas. this peak disappears andis replaced with a peak at 330 m in the 4-methoxyphenyl thiooxamic acidcompound. On completion of the phosphorus pentasulfide reaction, theentire reaction mixture is extracted with aqueous base such as aqueoussodium or potassium hydroxide. The 4-methoxyphenyl thiooxamic acid,represented by Formula III in the above flow diagram, is convenientlyrecovered in solid form by acidification of the alkaline extract. Thisproduct is highly pure and may be conveniently employed without furtherpurification in the next step of the reaction process. In addition to 4-methoxyphenyl thiooxamic acid, 4-ethoxyphenyl thiooxamic acid,4-isopropoxyphenyl thiooxamic acid and 2- methoxyphenyl thiooxamic acidare obtained in this manner from the appropriate starting materials.

The next step of our process comprises the formation of6-methoxybenZothiazole-2-carboxylic acid by oxidation of 4-methoxyphenylthiooxamic acid. For this purpose oxidizing agents such as bromine orferric chloride may be used, although an alkali metal ferricyanide suchas sodium or potassium ferricyanide is the preferred oxidizing agent.The reaction with ferricyanide is carried out in the presence of a basesuch as an alkali metal hydroxide, and it is convenient to use a diluteaqueous solution of sodium or potassium hydroxide as the solvent medium.For optimum results the temperature of the reaction mixture ismaintained below 25 C. and preferably in the range of 015 C. An alkalimetal salt of 6-methoxybenzothiazole 2 carboxylic acid precipitates fromthe reaction mixture and is conveniently converted to the free acid witha mineral acid such as hydrochloric, hydrobromic or sulfuric acid. Other6-loweralkoxybenzothiazole-2-carboxylic acids and the4-loweralkoxybenzothiazole-2-carboxylic acids are obtained in similarfashion from the thiooxarnic acid precursor.

I e The G-met'hoxybenzothiazole-2-carboxylic acid (IV,

A=H, B=OCH is converted to an ester in the next step of our synthesis.It is preferred to make a lower alkyl ester such as the methyl, ethyl,propyl, isopropyl or amyl ester although other esters such as a benzylester could be employed, if desired. Esterification may be effected bytreatment of the free acid with a diazoalkane in an anhydrous medium orby reaction of the acid with a lower alkanol in the presence of amineral acid. Thus, one convenient method involves intimately contacting6- methoxybenzothiazole-Z-carboxylic acid with a slight molar excess ofa diazoloweralkane such as diazomethane or diazoethane. This reaction ispreferably carried out in an ether medium at temperatures in the rangeof from about 5 C. to about 20 C. Alternatively, the free acid may bedissolved in alkanolic hydrogen chloride or hydrogen bromide and theresulting mixture allowed to stand for at least about 3 hours at roomtemperature or above. The ester thus formed is crystallized by coolingthe solution to about 0 C. or below. It is conveniently purified byrecrystallization from a lower alkanol.

The succeeding reaction involves conversion of the ester V to6-methoxybenzothiazole-2-carboxamide (VI), a process which is broughtabout by treating a lower alkanolic solution of the ester with ammoniaat elevated temperatures. An excess of ammonia is normally used and thereaction allowed to proceed for from 15 to 120 minutes, at or near threflux temperature of the alcoholic solvent. The desired amide isinsoluble in the reaction medium and is conveniently recovered bycooling of the mixture and filtration. Other amides of Formula VI, suchas 6-ethoxybenzothiazole-2-carboxamide,6-isopropoxybenzothiazole-Z-carboxamide and4-methoxybenzothiazole-2-carboxamide are obtained in similar fashionfrom the corresponding ester of Formula V. Although the preferred methodof obtaining these amides is from the ester as just described, they maybe produced, in accordance with an additional aspect of this invention,from the free acid IV or from the corresponding acid halide by reactionof such compounds with ammonia.

In the next step of our process, the amide is reacted with a phosphorusoxyhalide such as phosphorus oxychloride or phosphorus oxybromide at anelevated temperature for a short period of time. Under these conditionsthe amide is converted to 2-cyano-6-methoxybenzothiazole, the compoundof Formula VII above. The 2-cyano compound is recovered in substantiallypure form by decomposition of any excess phosphorus oxyhalide with adilute base, extraction into a water-immiscible organic solvent andchromatography using an adsorbent such as activated alumina. Theserecovery procedures may, of course, be modified as known to thoseskilled in the art and are not a critical aspect of the presentinvention. In addition to 2-cyano-6-methoxybenzothiazole,2-cyano-6-ethoxybenzothiazole, 2-cyano-6- isopropoxybenzothiazol and2-cyano-4-methoxybenzothiazole are examples of other compounds which arewithin the scope of our invention and which are obtained in the samemanner.

The 2-cyano-6-loweralkoxybenzothiazole or Z-cyano-4-loweralkoxybenzothiazole of Formula VII is then converted to2-cyano-6-hydroxybenzothiazole or 2-cyano-4- hydroxybenzothiazole (VIII)by cleavage of the ether function with pyridine hydrochloride. Thisether cleavage is conducted in a reaction medium comprising dry pyridineand under preferred conditions it is carried out at temperatures of fromISO-250 C. for 30-120 minutes. An excess of hydrogen chloride ismaintained in the medium. When the reaction is complete the 2-cyano-6-hydroxybenzothiazole or 2-cyano-4-hydroxybenzothiazole is convenientlyrecovered by neutralization of the reaction mixture with a weak basesuch as sodium carbonate or sodium bicarbonate. The benzothiazole isinsoluble at a slightly basic pH and is recovered by filtration andpurified by techniques such as chromatography and/or recrystallizationfrom organic solvents.

In the final step of our process the Z-cyanobenzothiazole VIII isreacted with a mercapto compound to give the 2-substituted benzothiazoleof Formula I. The particular mercapto compound employed will, of course,depend on the end product desired, and may be defined structurally asfollows:

H 113(1) (J-C O OH where x is CH C(CH or CH CH When cysteine isintimately contacted with 2-cyano-4-hydroxybenzothiazole or withZ-cyano-6-hydroxybenziothiazole, there is produced2-[2-(4'-carboxy)-thiazolinyl]-4-hydroxybenzothiazole or2-[2'-(4-carboxy)-triazolinyl] -6- hydroxybenzot-hiazole, respectively.Reaction of the 2- cyano-4(or 6)-hydroxy-benzothiazole withpenicillamine yields 2- [2- (4'-carboxy-5 ',5'-dimethyl -thiazolinyl] -4(or '6)-hydroxybenzothiazole, and treatment of the benzothiazole VIIIwith u-amino-v-mercapto butyric acid leads to formation of 2-[2'(4'-carboxy) 5,6 dihydro- 4H-1,3-thiazinyl]-4(or 6)-hydroxybenzothiazoleof the formula N J N Y \S where one of X and Y is a hydroxy group andthe other is hydrogen. This reaction is preferably conducted in-anoxygen-free atmosphere in order to minimize decomposition and formationof undesired by-products. Satisfactory results are obtained byintimately contacting the two reactants in an aqueous alcoholic reactionmedium such as aqueous methanol or ethanol at a slightly basic pH. Aswill be appreciated by those skilled in this art, the mercapto compoundemployed as one of the reactants has an asymmetric center and so mayexist in the D-, L- or DL- forms. This center of asymmetry is notdestroyed in the reaction so that substances of Formula I may beoptically active depending upon the optical activity of the reactant.For instance, when D-cysteine is reacted withZ-cyano-6-hydroxybenzothiazole, there is obtained 1-2- 2- (4carboxy)-thiaz-o1inyl] -6-hydroxybenzothiazole. correspondingly, the use of L-or DL-cysteine affords the dand dl-isomers.

The Z-substituted benzothiazoles of Formula I are chemiluminescent onoxidation, and are thus of value as sources of cold light. Thequaternary salts thereof are useful as optical sensitizers forphotographic silver halide emulsions in that they extend the sensitivityto the red, infra-red and far red regions of the spectrum. The 1-2-[2'-(4'-carboxy) -thiazolinyl]-6-hydroxybenzothiazole is useful in thedetermination of or assay for adenosine triphosphate (ATP) in substancescontaining undetermined amounts of ATP because of its property of bio-(EOOH luminescence in a system containing it, magnesium ion, oxygen, theenzyme luciferase and ATP.

The following examples are given for the purpose of illustration and notby way of limitation:

EXAMPLE 1 4-meth0xyphenyl thiooxamic acid 28 g. of ethylN-(4-methoxy)phenyl oxamate is dissolved in 560 ml. of boiling xylene.8.4 g. of phosphorous pentasulfide is added slowly to the refluxingsolution. The solution gradually turns black. Reflux is continued untilthe peak at 283 m in the ultraviolet spectrum has been replaced by apeak at 330 mu. (Forty minutes reflux is generally sufficient.) Thereaction mixture is then cooled and extracted with 5 x 200 ml. of l Nsodium hydroxide. The basic extracts are combined, filtered and cooledin ice. Crude 4-methoxyphenyl thiooxamic acid is precipitated by makingthe solution strongly acid with concentrated hydrochloric acid. Theorange-yellow precipitate'is collected and washed with cold water. Thismaterial is used without further purification in the next step. Onpurification by recrystallization from aqueous methanol it has M.P.134-8 C. (dec.). The ammonium salt melts at 180 C. (dec.).

Similar results are obtained When other lower alkyl N-(4-methoxy)phenyloxamates, such as methyl or propyl N-(4-methoxy)phenyl oxamate, areemployed as starting material in the above procedure. When ethyl N-(2-methoxy)phenyl oxamate is treated in this manner with phosphoruspentasulfide, 2-methoxyphenyl thiooxamic acid is produced.

EXAMPLE 2 6-methoxybenzothiazole-Z-carboxylic acid The 4-methoxyphenylthiooxamic acid obtained in Example l is dissolved in 400 ml. of l Nsodium hydroxide. This solution is added dropwise with continuousstirring to a solution prepared by dissolving 105 g. of potassiumferricyanide in 265 ml. of hot water and cooling to 8-10 C. Thetemperature of the reaction mixture is kept below 10 C. by cooling in anice bath. Fifteen minutes after addition of the thioacid is complete,the precipitated sodium salt of 6-methoxybenzothiazole-Z-carboxylic acidis removed by filtration and Washed with 0.5 N sodium hydroxide. Thissalt may be recrystallized from methanol. This crude salt is convertedto the free acid by slurrying in 500 ml. of water. After the slurry hasbeen freed of lumps, it is brought to pH 1 with concentratedhydrochloric acid. The solid 6-methoxybenzothiazole-2- carboxylic acidis collected, washed with a small amount of ice water and dried undervacuum, M.P. 109-111 C. The sodium salt obtained as the immediatereaction product may be purified by recrystallization from methanol.

4-methoxybenzothiazole-Z-carboxylic acid is made by the above processfrom 2-methoxyphenyl thiooxamic acid.

EXAMPLE 3 Methyl ester of 6-methoxybenzothiazoIe-Z-carboucylic acid (A)14 g. of 6-methoxybenzothiazole-2-carboxylic acid is slurried in 300 m1.of cold methanol. Ethereal diazomethane solution is added withcontinuous stirring until a slight excess is present. The reactionmixture is then allowed to stand in an ice bath for 15 minutes. Theether and excess diazomethane are then removed under vacuum, and themethyl ester of 6-methoxybenzothiazole- Z-carboxylic acid isprecipitated by addition of 500 ml. of water to the methanol solution.The product is collected and recrystallized from methanol usingdecolorizing charcoal, M.P. 142143 C.

(B) To a solution of 68.8 g. of 6-methoxybenzothiazole-Z-carboxylic acidin 5 l. of methanol there is added a solution of 135 g. of anhydroushydrogen chloride in 500 g. of methanol. The resulting solution isallowed to stand for 2 days at room temperature. It is then cooled 6 to10 C. for 15 hours and the resulting solid methyl ester of6-methoxybenzothiazole-Z-carboxylic acid collected by filtration. Aftertwo recrystallizations from methanol the melting point is 142142.8 C.

The methyl ester of 4-methoxybenzothiazole-2-carboxylic acid issimilarly obtained by treatment of the free acid with methanolichydrogen chloride or with diazomethane by the procedures set forthabove.

()ther lower alkyl esters, such as the ethyl, propyl, isopropyl and amylesters, are produced by using the appropriate lower alkanol ordiazoloweralkane in place of methanol or diazomethane.

EXAMPLE 4 6-meth0xybenz0thiwzole-Z-carbaxamide 7 g. of the methyl esterof 6-methoxybenzothiazole-2- carboxylic acid is dissolved in ml. ofboiling methanol. The hot solution is saturated with anhydrous ammoniaand kept hot for 30 minutes during which time 6-methoxybenzothiazole-2-carboxamide precipitates. The mixture is cooledand the amide collected. in almost quantitative yield by filtration.Recrystallization from acetic acid or pyridine gives substantially purematerial, M.P. 258-260 C. (dec.).

When methyl 4-methoxybenzothiazole-2-carboxylate is treated withanhydrous ammonia in the same way, 4- methoxybenzothiazole-Z-carboxamideis formed.

EXAMPLE 5 2-cyan0-6-methoxybenzothiazole 6.2 g. of6-methoxybenzothiazole 2 carboxamide is mixed with 30 ml. of phosphorusoxychloride and the mixture heated under gentle reflux until all theamide dissolves (about 15 minutes). Heating is continued for 3additional minutes. The solution is then cooled slightly and excessphosphorus oxychloride removed as completely as possible using a rotaryevaporator. The residue is cooled in ice and the residual phosphorusoxychloride decomposed with 10% NaHCO solution. A small amount of etheris added to prevent foaming. The pH of the slurry is adjusted to about6. The slurry is then extracted with chloroform and the chloroformextracts dried over magnesium sulfate and evaporated to dryness. Theresidual 2-cyano-6-methoxy-benzothiazole is purified by chromatographyover acid-washed alumina. A chloroform solution of 4 g. of the nitrileis evaporated with 50 g. of alumina and this added to a column of 300 g.of alumina packed in petroleum ether. Elution with ether/ petroleumether affords 2-cyano-6-methoxybenzothiazole, M.P. l29l31 C., afterrecrystallization from isooctane.

2-cyano-4-methoxybenzothiazole is obtained by treating4-methoxybenzothiazole-2-carboxamide with phosphorus oxychloride asdescribed in the preceding paragraph.

EXAMPLE 6 2-cyart0-6-hydr0xybenz0thiaz ole through a column of 3 g. ofactivated charcoal and 5.

g. of Celite and the column washed with ml. of ethanol. The eluates arecombined and concentrated to a volume of about 50 ml.2-cyano-6-hydroxybenzothiazole is precipitated therefrom by addition ofWater to the ethanolic concentrate. The product is recovered byfiltration and recrystallized from ethanol/water or ethyl acetate togive substantially pure material, M.P. 2l1- 213 C.

When 2-cyano-4-methoxybenzothiazole is treated in the same way,2-cyano-4-hydroxybenzothiazole is formed.

EXAMPLE 7 2- [2'- (4'-carb0xy -thiaz0linyl] -6-hydroxybenzothiazole 416mg. of D-cysteine hydrochloride monohydrate is dissolved in 8 ml. ofoxygen-free water. The pH of the solution is adjusted to about 8.5 with3.5 ml. of 1 N sodium hydroxide solution. This solution is then added toa stirred solution of 400 mg. of 2-cyano-6-hydroxybenzothiazole in 12ml. of methanol under nitrogen. A gelatinous precipitate forms whichdissolves in about 15 minutes. After the solution is stirred for 1 hour,it is filtered through Celite and acidified with 2 ml. of acetic acid.The solution is chilled for about 15 hours during which time 1,2[2'-(4'-carboxyythiaziolinyl]-6-hydroxybenzothiazole precipitates. Theproduct is recovered by filtration and dried, M.P. 196 C. (dec.); [u]=-29 (DMF). When L-cysteine hydrochloride and DL-cysteine hydrochlorideare employed in the above process, the dand dlforms of2-[2'-(4'-carboxy)-thiazolinyl]-6- hydroxybenzothiazole are produced.Correspondingly, reaction of 2-cyano-4-hydroxybenzothiazole withD-cysteine yields1-2-[2'-(4-carboxy)-thiazolinyl]-4-hydroxybenzothiazole.

EXAMPLE 8 When 324 mg. of a-amino-y-mercapto butyric acid or 360 mg. ofpencillamine are reacted with 400 mg. of 2- cyano-6-hydroxybenzothiazoleby the procedure of Example 7 there is obtained2-[2-(4-carboxy)-5,6-dihydro- 4H-l,3-thiazinyl] 6 hydroxybenzothiazoleor 2- [2'-(4- carboxy-',5-dimethyl)-thiazolinyl] 6hydroxy-benzothiazole, respectively. Use of2-cyano-4-hydroxy-benzothiazole in place of2-cyano-6-hydroxybenzothiazole affords the corresponding2-substituted-4-hydroxybenzothiazoles.

Any departure from the above description which conforms to the presentinvention is intended to be included Within the scope of the claims.

What is claimed is: 1. The process for preparing a compound of theformula i Y S /C N that comprises reacting a compounds of the formula BSLCONH:

where A and B are different members of the class consisting of hydrogenand lower alkoxy.

3. 2-cyano-6-methoxybenzothiazole.

4. A compound having the formula where X and Y are different members ofthe class consisting of hydrogen and hydroxy groups.

5. 2-cyano-6-hydroxybenzothiazole.

6. Z-cyano-4-hydroxybenzothiazole.

No references cited.

ALEX MAZEL, Primary Examiner.

ALTON D. ROLLINS, Assistant Examiner.

1. THE PROCESS FOR PREPARING A COMPOUND OF THE FORMULA
 2. A COMPOUNDSHAVING THE FORMULA
 4. A COMPOUND HAVING THE FORMULA